A few suitable tankmates
Synodontis Catfish - 7" (very active at night)
Black Lancer Catfish - 8"
Clown Loach - 12" (should be kept in groups of at least four) would advise against as you tank will be 2 small
Silver Dollars - 6" (also like being in groups)
Severum - 12" (excellent tank mates for Oscars )
Jade Eyed Cichlid - 6" (excellent tank mates for Oscars)
Chocolate Cichlid - 12"
Pumpkinseed - 5 to 7"
Banded Leporinus - 12" (can occasionally nip at other fishes fins)
Satanoperca jurupari (Earth-Eater) - 10" (very placid & like being in groups)
Blue Acara - 8" (get more aggressive the older they get)
some ideas.... but remeber that your oscar will grow quickly and some of these grow quite slow, if you can manage to get a larger tank then do so then you can have a few more fish!